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At large scales, particulate suspensions flow like homogeneous viscous liquids, but at the particle scale, the role of the local heterogeneity brought by the particles cannot be neglected. The volume fraction also matters; in dense suspensions, particulate effects can be felt across distances much larger than the particle diameter. Therefore, whether a suspension should behave as a homogeneous or heterogeneous fluid is a matter of scale. Here, we consider the canonical situation of the pinch-off of suspension drops to study the behavior of suspensions at different scales. Initially, the filament of suspension thins down like a homogeneous liquid until reaching a critical thickness at which the thinning accelerates. Eventually, a region devoid of particles appears, and the breakup occurs similarly to a homogeneous viscous liquid. Although this problem have been studied for almost 20 y, the role of heterogeneity in the acceleration of the pinch-off is still not understood. We show that the onset of heterogeneity corresponds to the dislocation of the suspensions where local fluctuations in particle concentration increase. We derive scaling laws for the dynamics in the heterogeneous regime and develop a model to predict the coherence length at which the discrete nature of the particles appears, and we demonstrate that this length depends both on the particle size and on the volume fraction of the suspension. We extend this approach to polydisperse suspensions. Our work sheds light on the mesoscopic scale below which starts the heterogeneous regime and a continuum approach is not valid anymore. [ABSTRACT FROM AUTHOR]

Les tsunamis générés par des glissements de terrains ou la chute de pans de montagnes dans les océans, lacs ou fleuves, peuvent être particulièrement destructeurs et meurtriers pour les côtes avoisinantes. Ces évènements ont été dernièrement reproduits en laboratoire, dans une configuration modèle semblable à celle d'un château de sable s'écroulant dans l'eau sous l'effet de la gravité. [ABSTRACT FROM AUTHOR]

Le comportement des matériaux granulaires humides hétérogènes, éléments incontournables de nombreux processus industriels et géophysiques, résiste encore à une modélisation complète. L'interaction des grains avec une phase liquide doit être éclaircie afin d'optimiser les procédés de mélange, notamment la dispersion des grains d'un agrégat cohésif. Modéliser ces interactions permettrait aussi de mieux décrire des éboulements granulaires dont les propriétés mécaniques évoluent avec des variations de teneur en liquide et de répartition spatiale. Des recherches actuelles se tournent vers la compréhension des échanges dans ces matériaux, pour isoler les mécanismes par lesquels des grains se mélangent à un liquide ou à une phase granulaire humide. [ABSTRACT FROM AUTHOR]

We characterize the different morphologies adopted by a drop of liquid placed on two randomly oriented fibers, which is a first step toward understanding the wetting of fibrous networks. The present work reviews previous modeling for parallel and touching crossed fibers and extends it to an arbitrary orientation of the fibers characterized by the tilting angle and the minimum spacing distance. Depending on the volume of liquid, the spacing distance between fibers and the angle between the fibers, we highlight that the liquid can adopt three different equilibrium morphologies: 1) a column morphology in which the liquid spreads between the fibers, 2) a mixed morphology where a drop grows at one end of the column or 3) a single drop located at the node. We capture the different morphologies observed using an analytical model that predicts the equilibrium configuration of the liquid based on the geometry of the fibers and the volume of liquid. Graphical abstract: [ABSTRACT FROM AUTHOR]

Weinvestigate experimentally the drying of a small volume of perfectlywetting liquid on two crossed fibers. We characterize the drying dynamicsfor the three liquid morphologies that are encountered in this geometry:drop, column, and a mixed morphology, in which a drop and a column coexist. Foreach morphology, we rationalize our findings with theoretical modelsthat capture the drying kinetics. We find that the evaporation ratesignificantly depends upon the liquid morphology and that the dryingof the liquid column is faster than the evaporation of the drop andthe mixed morphology for a given liquid volume. Finally, we illustratethat shearing a network of fibers reduces the angle between them,changes the morphology toward the column state, and therefore, enhancesthe drying rate of a volatile liquid deposited on it. [ABSTRACT FROM AUTHOR]

We report on a microfluidic method that allows measurement of a small concentration of large contaminants in suspensions of solid micrometer-scale particles. To perform the measurement, we flow the colloidal suspension through a series of constrictions, i.e., a microchannel of varying cross-section. We show and quantify the role of large contaminants in the formation of clogs at a constriction and the growth of the resulting filter cake. By measuring the time interval between two clogging events in an array of parallel microchannels, we are able to estimate the concentration of contaminants whose size is selected by the geometry of the microfluidic device. This technique for characterizing colloidal suspensions offers a versatile and rapid tool to explore the role of contaminants on the properties of the suspensions. [ABSTRACT FROM AUTHOR]

Whether it's pipes, highways, or arteries that are clogged, stopping the flow is always inconvenient and sometimes dangerous. [ABSTRACT FROM AUTHOR]

Immiscible aqueous phases, formed by dissolving incompatible solutes in water, have been used in green chemical synthesis, molecular extraction and mimicking of cellular cytoplasm. Recently, a microfluidic approach has been introduced to generate all-aqueous emulsions and jets based on these immiscible aqueous phases; due to their biocompatibility, these all-aqueous structures have shown great promises as templates for fabricating biomaterials. The physico-chemical nature of interfaces between two immiscible aqueous phases leads to unique interfacial properties, such as an ultra-low interfacial tension. Strategies to manipulate components and direct their assembly at these interfaces needs to be explored. In this paper, we review progress on the topic over the past few years, with a focus on the fabrication and stabilization of all-aqueous structures in a multiphase microfluidic platform. We also discuss future efforts needed from the perspectives of fluidic physics, materials engineering, and biology for fulfilling potential applications ranging from materials fabrication to biomedical engineering. [ABSTRACT FROM AUTHOR]

We study numerically the dynamics of jets and drops in a microcapillary co-flow device. The co-flow stream encounters different flow regimes, including dripping, jetting. Using a level-set/finite element axysimmetric numerical simulation, we study the dynamics of breakup of a jet subject to flow rate perturbations. A scaling law for the width of the unperturbed jet is presented and compared to existing experimental results as well as numerical measurements. Then, we show that the introduction of a sinusoidal perturbation of the inner fluid flow rate can facilitate breakup of the fluid in regimes where a jet is usually observed. Moreover, the flow rate perturbation leads to a good control over the size and the frequency of the resulting droplets. Using theoretical modelling, we provide a criterion to determine the optimal frequency to break up the jet. We also derive scaling laws to determine the volume of inner fluid encapsulated in the emulsion droplets as a function of the frequency and to estimate the distance for the jet to break up as a function of the amplitude of perturbation. These scaling laws are in good agreement with results of numerical simulations. Our work suggests a novel approach and offers guiding principles to break up liquid jets in cases where dripping is difficult to achieve. [ABSTRACT FROM AUTHOR]

We report an easy-to-implement method that allows the direct generation of water-in-water (w/w) single emulsions. The method relies on direct perturbation of the pressure that drives the flow of the dispersed phase of the emulsions. The resultant inner jet is induced to break up into droplets due to the growth of the perturbation through Rayleigh-Plateau instability [L. Rayleigh, Proc. R. Soc. London 29, 71-97 (1879)]; this leads to the formation of monodisperse droplets. By implementing this method on a modified microfluidic device, we directly generate water-in-water-in-water (w/w/w) double emulsions with good control over the size and the number of encapsulated droplets. Our approach suggests a route to apply droplet-based microfluidics to completely water-based systems. [ABSTRACT FROM AUTHOR]

The flow in a cylinder driven by time-harmonic oscillations of the rotation rate, called longitudinal librations, is investigated. Using a theoretical approach and axisymmetric numerical simulations, we study two distinct phenomena appearing in this librating flow. First, we investigate the occurrence of a centrifugal instability near the oscillating boundary, leading to the so-called Taylor-Görtler vortices. A viscous stability criterion is derived and compared to numerical results obtained for various libration frequencies and Ekman numbers. The strongly nonlinear regime well above the instability threshold is also documented. We show that a new mechanism of spontaneous generation of inertial waves in the bulk could exist when the sidewall boundary layer becomes turbulent. Then, we analyse the librating flow below the instability threshold and characterize the mean zonal flow correction induced by the nonlinear interaction of the boundary layer flow with itself. In the frequency regime where inertial modes are not excited, we show that the mean flow correction in the bulk is a uniform rotation, independent of the Ekman number and cylinder aspect ratio, in perfect agreement with the analytical results of Wang ['Cylindrical tank of fluid oscillating about a state of steady rotation,' J. Fluid. Mech. 41, 581 (1970)]. When inertial modes are resonantly excited, the mean flow correction is found to have a more complex structure. Its amplitude still scales as the square of the libration amplitude but now depends on the Ekman number. [ABSTRACT FROM AUTHOR]

Microfluidic devices can be used to produce highly controlled and monodisperse double or multiple emulsions. The presence of inner drops inside a jet of the middle phase introduces deformations in the jet, which leads to breakup into monodisperse double emulsions. However, the ability to generate double emulsions can be compromised when the interfacial tension between the middle and outer phases is low, leading to flow with high capillary and Weber numbers. In this case, the interface between the fluids is initially deformed by the inner drops but the jet does not break into drops. Instead, the jet becomes highly corrugated, which prevents formation of controlled double emulsions. We show using numerical calculations that the corrugations are caused by the inner drops perturbing the interface and the perturbations are then advected by the flow into complex shapes. [ABSTRACT FROM AUTHOR]

Hypothesis: The dip coating of suspensions made of monodisperse non-Brownian spherical particles dispersed in a Newtonian fluid leads to different coating regimes depending on the ratio of the particle diameter to the thickness of the film entrained on the substrate. In particular, dilute particles dispersed in the liquid are entrained only above a threshold value of film thickness. In the case of anisotropic particles, in particular fibers, the smallest characteristic dimension will control the entrainment of the particle. Furthermore, it is possible to control the orientation of the anisotropic particles depending on the substrate geometry. In the thick film regime, the Landau-Levich-Derjaguin model remains valid if one account for the change in viscosity. Experiment: To test the hypotheses, we performed dip-coating experiments with dilute suspensions of non-Brownian fibers with different length-to-diameter aspect ratios. We characterize the number of fibers entrained on the surface of the substrate as a function of the withdrawal velocity, allowing us to estimate a threshold capillary number below which all the particles remain in the liquid bath. Besides, we measure the angular distribution of the entrained fibers for two different substrate geometries: flat plates and cylindrical rods. We then measure the film thickness for more concentrated fiber suspensions. Findings: The entrainment of the fibers on a flat plate and a cylindrical rod is primarily controlled by the smaller characteristic length of the fibers: their diameter. At first order, the entrainment threshold scales similarly to that of spherical particles. The length of the fibers only appears to have a minor influence on the entrainment threshold. No preferential alignment is observed for non-Brownian fibers on a flat plate, except for very thin films, whereas the fibers tend to align themselves along the axis of a cylindrical rod for a large enough ratio of the fiber length to the radius of the cylindrical rod. The Landau-Levich-Derjaguin law is recovered for more concentrated suspension by introducing an effective capillary number accounting for the change in viscosity. [ABSTRACT FROM AUTHOR]

Wet granular aggregates are common precursors of construction materials, food, and health care products. The physical mechanisms involved in the mixing of dry grains with a wet substrate are not well understood and difficult to control. Here, we study experimentally the accretion of dry grains on a wet granular substrate by measuring the growth dynamics of the wet aggregate. We show that this aggregate is fully saturated and its cohesion is ensured by the capillary depression at the air-liquid interface. The growth dynamics is controlled by the liquid fraction at the surface of the aggregate and exhibits two regimes. In the viscous regime, the growth dynamics is limited by the capillary-driven flow of liquid through the granular packing to the surface of the aggregate. In the capture regime, the capture probability depends on the availability of the liquid at the saturated interface, which is controlled by the hydrostatic depression in the material. We propose a model that rationalizes our observations and captures both dynamics based on the evolution of the capture probability with the hydrostatic depression. [ABSTRACT FROM AUTHOR]

Physical and chemical transformation processes in reactive granular media involve the reorganization of the structure. In this paper, we study experimentally the rearrangements of a two-dimensional (2D) granular packing undergoing a localized transformation. We track the position and evolution of all the disks that constitute the granular packing when either a large intruder shrinks in size or is pulled out of the granular structure. In the two situations the displacements at long time are similar to 2D quasistatic silo flows whereas the short-time dynamic is heterogeneous in both space and time. We observe an avalanchelike behavior with power-law distributed events uncorrelated in time. In addition, the instantaneous evolutions of the local solid fraction exhibit self-similar distributions. The averages and the standard deviations of the solid fraction variations can be rescaled, suggesting a single mechanism of rearrangement. [ABSTRACT FROM AUTHOR]

Debris flows are dense and fast-moving complex suspensions of soil and water that threaten lives and infrastructure. Assessing the hazard potential of debris flows requires predicting yield and flow behavior. Reported measurements of rheology for debris flow slurries are highly variable and sometimes contradictory due to heterogeneity in particle composition and volume fraction (φ) and also inconsistent measurement methods. Here we examine the composition and flow behavior of source materials that formed the post-wildfire debris flows in Montecito, CA, in 2018, for a wide range of φ that encapsulates debris flow formation by overland flow. We find that shear viscosity and yield stress are controlled bythe distance fromjamming, Δφ = φm - φ, where the jamming fraction 0m is a material parameter that depends on grain size polydispersity and friction. By rescaling shear and viscous stresses to account for these effects, the data collapse onto a simple nondimensional flow curve indicative of a Bingham plastic (viscoplastic) fluid. Given the highly nonlinear dependence of rheology on Δφ, our findings suggest that determining the jamming fraction for natural materials will significantly improve flow models for geophysical suspensions such as hyperconcentrated flows and debris flows. [ABSTRACT FROM AUTHOR]

Transport of liquid drops in fibrous media occurs in various engineering systems such as fog harvesting or cleaning of textiles. The ability to tune or to control liquid movement can increase the system efficiency and enable new engineering applications. In this letter, we experimentally investigate how partially wetting drops on a single fiber can be manipulated by vibrating the fiber. We show that a sliding motion along the fiber or a dripping of the drop can be triggered by standing waves. We identify the conditions on the drop volume, the fiber tilt angle, and the amplitude and frequency of oscillations to observe these different behaviors. Finally, we experimentally illustrate that vibrations can be used to control the transport and the collection of water drops along a fiber using a combination of the sliding and dripping transitions. [ABSTRACT FROM AUTHOR]

[Display omitted] The coalescence of bare droplets when surface tension dominates always results in one larger spherical droplet. In contrast, droplets coated with particles may be stabilized into non-spherical structures after arrested coalescence, which can be achieved by different approaches, such as changing the particle surface coverage. The size of particles coating the initial liquid marbles can be used to control the coalescence dynamics and the resulting morphology of arrested droplets. We characterized the electrocoalescence of liquid marbles coated with particles ranging from hundred nanometers to hundred micrometers. The electrocoalescence was recorded using high-speed imaging. When the electrocoalescence initiates, particles jam and halt the relaxation of the marbles at different stages, resulting in four possible final morphologies that are characterized using the Gaussian curvature at the neck region. The four regimes are total coalescence, arrested puddle coalescence, arrested saddle coalescence, and non-coalescence. The coalescence is initiated at the center of the contact zone, independent of the particle size. Small particles show little resistance to the coalescence, while marbles coated by large particles demonstrate a viscous-like behavior, indicated by the growth of the liquid bridge and the damping. The present study provides guidelines for applications that involve the formulation of liquid marbles with complex morphologies. [ABSTRACT FROM AUTHOR]

We report a new method to display the minute fluctuations induced by syringe pumps on microfluidic flows by using a liquid–liquid system with an ultralow interfacial tension. We demonstrate that the stepper motor inside the pump is a source of fluctuations in microfluidic flows by comparing the frequencies of the ripples observed at the interface to that of the pulsation of the stepper motor. We also quantify the fluctuations induced at different flow rates, using syringes of different diameters, and using different syringe pumps with different advancing distances per step. Our work provides a way to predict the frequency of the fluctuation that the driving syringe pump induces on a microfluidic system and suggests that syringe pumps can be a source of fluctuations in microfluidic flows, thus contributing to the polydispersity of the resulting droplets. [ABSTRACT FROM AUTHOR]

Modeling of tsunami waves generated by subaerial landslides is important to provide accurate hazard and risk assessments in coastal areas. We perform small-scale laboratory experiments where a tsunami-like wave is generated by the gravity-driven collapse of a subaerial granular column into water. We show that the maximal amplitude reached near-shore by the generated wave in our experiments is linked to the instantaneous immersed volume of grains and to the ultimate immersed deposit. Despite the differences in scale and geometry between our small-scale experiments and the larger-scale geophysical events, a rather good agreement is found between the experimental law and the field data. This approach offers an easy way to estimate the amplitude of paleo-tsunamis. [ABSTRACT FROM AUTHOR]

We present a numerical method for the solution of interfacial growth governed by the Stefan model coupled with incompressible fluid flow. An algorithm is presented which takes special care to enforce sharp interfacial conditions on the temperature, the flow velocity and pressure, and the interfacial velocity. The approach utilizes level-set methods for sharp and implicit interface tracking, hybrid finite-difference/finite-volume discretizations on adaptive quadtree grids, and a pressure-free projection method for the solution of the incompressible Navier-Stokes equations. The method is first verified with numerical convergence tests using a synthetic solution. Then, computational studies of ice formation on a cylinder in crossflow are performed and provide good quantitative agreement with existing experimental results, reproducing qualitative phenomena that have been observed in past experiments. Finally, we investigate the role of varying Reynolds and Stefan numbers on the emerging interface morphologies and provide new insights around the time evolution of local and average heat transfer at the interface. • Sharp numerical solver for the solution of Stefan problems with convective effects. • Finite-difference/finite-volume, level-set, and pressure-free projection methods. • Efficient computation using adaptive quadtree grids. • Apply sharp boundary conditions and calculate accurate gradients at the interface. • Validation/investigation of interface morphology and interfacial heat transfer. [ABSTRACT FROM AUTHOR]

The formation of gas bubbles in a liquid occurs in various engineering processes, such as during foam generation or agitation and mixing in bubbly flows. A challenge in describing the initial formation of a gas bubble is due to the singular behavior at pinch-off. Past experiments in Newtonian fluids have shown that the minimum neck radius follows a power-law evolution shortly before the break-up. The exponent of the power-law depends on the viscosity of the surrounding Newtonian liquid, and ranges from 0.5 for low viscosity to 1 for large viscosity. However, bubble formation in a viscoelastic polymer solution remains unclear, and in particular, if the evolution is still captured by a power-law and how the exponent varies with the polymer concentration. In this study, we use high-speed imaging to analyze the bubble pinch-off in solutions of polymers. We characterize the time evolution of the neck radius when varying the polymer concentration and thus the characteristic relaxation time of the polymer chains, and describe the influence of viscoelasticity on the bubble pinch-off. Our results reveal that the presence of polymers does not influence the thinning until the latter stages, when their presence in sufficient concentration delays the pinch-off. • The influence of viscoelasticity on the bubble pinch-off is investigated. • Shortly before the pinch-off the minimum neck radius follows a power-law evolution. • In Newtonian fluids the exponent of the power-law ranges from 0.5 to 1. • For dilute polymer solutions the exponent is constant and equal to 0.5. • The presence of polymers modified the shape of the interface of the bubble. [ABSTRACT FROM AUTHOR]

Abstract: Several studies have already considered the influence of tides on the evolution of systems composed of a star and a close-in companion to tentatively explain different observations such as the spin-up of some stars with hot Jupiters, the radius anomaly of short orbital period planets and the synchronization or quasi-synchronization of the stellar spin in some extreme cases. However, the nature of the mechanism responsible for the tidal dissipation in such systems remains uncertain. In this paper, we claim that the so-called elliptical instability may play a major role in these systems, explaining some systematic features present in the observations. This hydrodynamic instability, arising in rotating flows with elliptical streamlines, is suspected to be present in both planet and star of such systems, which are elliptically deformed by tides. The presence and the influence of the elliptical instability in gaseous bodies, such as stars or hot Jupiters, are most of the time neglected. In this paper, using numerical simulations and theoretical arguments, we consider several features associated to the elliptical instability in hot-Jupiter systems. In particular, the use of ad hoc boundary conditions makes it possible to estimate the amplitude of the elliptical instability in gaseous bodies. We also consider the influence of compressibility on the elliptical instability, and compare the results to the incompressible case. We demonstrate the ability for the elliptical instability to grow in the presence of differential rotation, with a possible synchronized latitude, provided that the tidal deformation and/or the rotation rate of the fluid are large enough. Moreover, the amplitude of the instability for a centrally-condensed mass of fluid is of the same order of magnitude as for an incompressible fluid for a given distance to the threshold of the instability. Finally, we show that the assumption of the elliptical instability being the main tidal dissipation process in eccentric inflated hot Jupiters and misaligned stars is consistent with current data. [Copyright &y& Elsevier]